1. Field of the Invention
This invention relates generally to pipe clamps and, more directly, is particularly suited for repairing jet pump riser assemblies within a boiling water nuclear reactor pressure vessel.
2. Description of the Prior Art
A reactor pressure vessel of a boiling water reactor typically has a generally cylindrical shape and is closed at both ends by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the reactor pressure vessel. A core shroud typically surrounds a core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. The top guide includes several openings through which fuel bundles are inserted with the fuel bundles supported by the core plate.
Water enters the reactor pressure vessel through an inlet nozzle in the reactor pressure vessel sidewall. A jet pump riser assembly is coupled to the inlet nozzle and the jet pump riser assembly typically includes, in part, a thermal sleeve, a lower elbow and a riser pipe. The thermal sleeve extends through the inlet nozzle and is welded to one end of the elbow. In some plants an extension, also known as a pup is welded between the thermal sleeve and the elbow. The other end of the elbow is welded to the riser pipe. The riser pipe is positioned between and extends substantially parallel to the reactor pressure vessel sidewall and the shroud in the relatively narrow space between the reactor pressure vessel and the shroud.
During operation, the jet pump assembly directs recirculation flow within the reactor pressure vessel. Accordingly, substantial amounts of water are forced through the jet pump assembly and significant hydraulic forces and vibration are exerted on the jet pump assembly, including on the jet pump assembly elbow. Furthermore, and during operation, a large amount of heat is generated within the reactor pressure vessel, thus placing thermal stress on the jet pump assembly. These hydraulic forces, vibration stresses, and thermal stresses might cause stress corrosion cracking in the riser pipe elbow, and in extreme cases, the riser pipe elbow could separate from the thermal sleeve which is undesirable.
Stress corrosion cracking is a known phenomenon occurring in reactor components, such as structural members, piping, fasteners, and welds, exposed to high temperature water. The reactor components are subject to a variety of stresses associated with, e.g., differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stresses from welding, cold working and other inhomogeneous metal treatments. In addition, water chemistry, welding, heat treatment and radiation can increase the susceptibility of metal in a component to exhibit stress corrosion cracking.
It would be desirable to provide an apparatus which provides support for a jet pump riser elbow, and, more particularly, the connection between the riser elbow and the thermal sleeve. Preferably, such an apparatus would maintain axial pressure between the riser elbow and the thermal sleeve to minimize leakage should the weld therebetween fail. Additionally, it would also be desirable to provide such an apparatus which is easy to install remotely as a repair in the limited access space available.
A number of clamp designs have attempted to address these issues, for example U.S. Pat. Nos. 6,053,652, 6,086,120, 6,108,391, 6,264,203 and 6,375,230. Further improvement is still desired to control the amount of any leakage and ease installation in a very confined area. In addition, it is desired to provide such support for both the weld between the thermal sleeve and the extension and the weld between the extension and the elbow in those plants that employ an extension. Further, it is desirable to provide such support that will take into consideration the reduced thickness of the elbow as compared to the thermal sleeve.